Cellular mechanisms underlying neurotoxic actions of methylmercury (MeHg) in the peripheral and central nervous system will be studied in mammals using intracellular microelectrode recording techniques and radiochemical flux determinations. The objectives are to determine: a) whether MeHg produces changes in synaptic transmission in cortical neurons which correspond to those observed in peripheral, somatic nerves; b) whether MeHg blocks nerve Ca2+ channels, and if so whether the ionic selectivity of the channel is altered, whether the effects are preferential for a particular type of Ca2+ channel, and whether congeners of MeHg produce similar effects; or whether MeHg blocks Ca2+ uptake or induces Ca2+ release from neuronal mitochondria and/or smooth endoplasmic reticulum; d) whether MeHg enters the nerve terminal, if so, by what path it enters, and where it goes; e) whether MeHg produces postsynaptic effects at the neuromuscular junction; f) to what extent do the electrophysiological changes described for MeHg occur when a permanently uncharged organic, or a monovalent inorganic form of Hg is used; and g) whether electrophysiological changes described with acute application of MeHg occur when animals are treated chronically with MeHg? Intracellular microelectrode recordings including voltage clamp and quantitative iontophoresis will be made from the somatic neuromuscular junction of the rat and from mossy fiber/CA3 pyramidal cell excitatory synapses in isolated hippocampal slices of the guinea pig. Effects of MeHg on nerve-evoked and spontaneous release will be measured. Effects of MeHg on Ca2+- driven action potentials in hippocampal cells will be studied. Effects of MeHg on Ca2+ currents in N1E-115 neuroblastoma cells using whole cell patch voltage clamp will be used to determine if MeHg blocks Ca2+ channels. Effects of MeHg on Ca2+ channel ion selectivity will be assessed using uptake of radiolabelled Ca, Sr and Ba. Comparative effects of structurally-related mercurials on calcium influx will also be studied. Synaptosomes will also be used to test whether MeHg enters the nerve terminal, and if so how MeHg interacts with intracellular Ca2+ regulatory systems in the nerve terminal. Measurements of nerve-evoked and spontaneous release from motor nerves of rats treated chronically with MeHg will be used to assess the relevance to chronic MeHg neurotoxicity of the changes described previously for acute intoxication. Results from the proposed studies should provide answers to questions concerning the interaction of MeHg with cortical synaptic transmission, the interaction, with Ca2+ in synaptic transmission at both membrane and intracellular loci, and the effects of chronic MeHg poisoning on synaptic transmission.